a) Field of the Invention
The present invention relates to the performance and use of transformers in telephone terminal equipment and, more particularly, an arrangement and technique for improving circuit performance and reducing the size of such transformers.
b) Background of the Prior Art
Any terminal device connected to a telephone line must exhibit isolation between that line and all other connections to the terminal equipment. This requirement, which is imposed by telecommunications regulatory agencies worldwide, serves both a network protection function and a performance assurance function. The common means of achieving this isolation is by coupling signals to and from the telephone line through a transformer.
Use of a transformer, while allowing the isolation requirement to be met, generally creates various technical problems which must be considered in the design of the terminal. Among these are:
1. Poor frequency response. The ability to transfer power from one winding of a transformer to another deteriorates as frequency is reduced.
2. Phase shifts, most significant at low frequencies, render hybrid cancellation, an important function of many terminal devices, difficult to achieve. Hybrid cancellation is achieved by subtracting the signal transmitted to the phone line from that reflected from the phone line. If the relative phase of these signals is not constant, the phase of one or the other of them must be shifted so as to neutralize the effect of the phase shift introduced by the transformer. Because this shift varies with frequency, this is a goal that can at best be poorly approximated.
3. Impedance reflected through the transformer cannot be directly correlated to the impedance with which the transformer is terminated. In particular, every country's telecommunications regulatory agency specifies that a terminal device exhibit some specific impedance to the telephone line. Selecting the impedance with which the transformer must be terminated so as to reflect the desired impedance to the line is a difficult engineering task. The usual result is that the desired impedance is only poorly approximated.
4. All of the above problems are compounded by the fact that the transformer's behavior is different for differing telephone line currents. As the DC current through the transformer winding facing the telephone loop increases, the transformer core approaches a condition of saturation. This reduces the mutual inductance of transformer windings, deteriorating the transformer's ability to convey power from one winding to another and magnifying the effects of the above problems.
Problems 1 through 3 above can be ameliorated to some extent by increasing the inductance of the transformer's windings and the coupling between them. Problem 4 can be reduced by incorporating more core material (usually some iron alloy) so that the core remains farther from saturation over the expected range of loop currents. These measures in turn introduce an additional problem which is that:
5. Good telecommunications transformers are physically large and heavy, and are expensive.
It is an object of the present invention to provide an arrangement and technique which substantially reduces problems 1 through 4 above, while allowing the use of a transformer that is smaller, lighter and cheaper than would otherwise be possible.